The practical application of photonic crystals, especially those with band gaps located in the visible regime, has been limited by the low efficiency and high cost involved in the conventional lithographic fabrication techniques. The fabrication challenges have provided a major driving force for study of alternative approaches to photonic crystal preparation. Indeed, many self-assembly processes have been successfully developed in the past two decades to organize uniform colloidal objects into ordered structures that show photonic response in the visible spectrum. Typical self-assembly methods include those utilizing gravitational force, centrifugal force, hydrodynamic flow, electrophoretic deposition, capillary force, and electrostatic interaction to assemble colloidal crystals. However, there are still challenges that need to be addressed before the self-assembly approaches can be widely used for fabricating photonic materials in an efficient manner. A major problem is in the fabrication efficiency: the formation of high quality colloidal crystals over a large area usually takes hours to days or even months to complete. The low production efficiency makes many applications impractical.
Recently, it was discovered that nanostructured superparamagnetic magnetite (Fe3O4) particles can be conveniently assembled under the external magnetic field to instantly produce ordered one-dimensional (1D) photonic structures, as driven by the balanced interaction of the induced magnetic attraction and various repulsions among the magnetite particles. Since there are many more choices for nonmagnetic colloidal particles with uniform sizes and optimal refractive indices, it would be advantageous to extend the magnetic assembly strategy to nonmagnetic particles to allow their rapid assembly into large-area photonic crystals with high quality. Conventionally, magnetic assembly of nonmagnetic materials is achieved by modifying these building blocks with magnetic materials, which apparently limits the choices of materials and the applicability of the processes. Accordingly, it would be desirable to demonstrate the use of nanocrystal-based ferrofluids to direct the assembly of nonmagnetic colloidal particles into photonic crystal structures. The process is general, efficient, convenient, and scalable, thus represents a new and practical platform for the fabrication of colloidal crystal-based photonic devices.